The present invention relates generally to a lighting application, and more particularly to a light source comprising an organic light emitting device with improved efficiency.
An organic light emitting device (OLED) is a type of a light emitting diode that emits light in response to an applied potential. A typical OLED comprises an anode, one or more organic material layers and a cathode. Cathodes generally comprise a material having a low work function such that a relatively small voltage causes the emission of electrons. Some commonly used material include calcium and metals, such as gold, indium, manganese, tin, led, aluminum, silver, magnesium, a silver/magnesium alloy or combinations thereof. Such materials, although having a low work function, exhibit relatively low melting points and/or exhibit high degradation when exposed to oxygen or water. Anodes generally comprise a transparent material having high work function value such as indium tin oxide (ITO), tin oxide, nickel, or gold. A layer of molybdenum oxide (MoO3) may be included to reduce the overall driving voltage.
One of the layers of the OLED comprises a material having the ability to transport holes, and is referred to as the hole transport layer. Another layer typically comprises a material having the ability to transport electrons, known as the electron transport layer. This layer may also function as the luminescent material (or emission layer) or an additional independent layer may be disposed between the hole transport layer and the electron transport layer. When a voltage is applied, a current of electrons flow through the device from the cathode to the anode. The anode injects positive charges (holes) into the hole transport layer, while the cathode injects negative charges (electrons) into the electron transport layer. Electrostatic forces bring the electrons and the holes together and they recombine near the light emitting layer, which causes a drop in energy levels and an emission of radiation in the range of visible light.
Significant efforts have been made in selecting materials and forming modified layer structures or materials in OLEDs to achieve improved performance. Numerous OLEDs with alternative layer structures have been disclosed. For example, OLEDs have been created containing additional functional layers. Some of these new layer structures with new materials have indeed resulted in improved device performance.
One OLED variation is illustrated, for example, in Toshinori Matsushima et al., “Marked improvement in electroluminescence characteristics of organic light-emitting diodes using an ultra-thin hole-injection layer of molybdenum oxide,” 104 Journal of Applied Physics 1-6 (2008). The OLED includes an aluminum cathode, an electron-injection layer comprising lithium fluoride, a light emitting electron-transport layer of tris(8-hydroxyquinoline) aluminum (III) (Alq3), a naphthyl-substituted benzidine derivative (α-NPD) hole transport layer, a molybdenum oxide (MoO3) hole injecting layer and an indium tin oxide (ITO) anode. The MoO3 thickness was said to be optimized to provide the lowest driving voltage, highest power conversion efficiency, and longest lifetime for the OLED. However, this configuration, although an improvement for the specific materials used, did not address some significant issues facing OLEDs. In particular, the Alq3 and α-NPD layers display narrow emission ranges and the OLEDs as a whole display low injection efficiency when the electron comes from an aluminum cathode. Furthermore, the interface coupling between ITO and MoO3 is weak, further lowering overall efficiency of the OLED.
Therefore, there is a continued need to improve OLED structure and further enhance the performance and efficiency of an OLED for use as a light source.